Squirrel-cage motor



Patented Sept. 15, 1931 UNITED STATES PATENT OFFICE ROBERT R. SHEELY, OF 'WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION F PENNSYLVANIA SQUIRREL-CAGE MOTOR Application `filed December 14, 1928. Serial No. 326,025.

45I rent, together with a high starting-torque and a low full-load slip; and it aims to attain the foregoing object in the most economical manner without resorting to a double-deck, squirrel-cage construction or to a special cross-section of squirrel-cage bars to approximate a double-deck construction and, as already indicated, without resorting to any parts which must change position between starting and full-speed or any parts which involve a material increase in the cost of manufacture.

The single figure of the drawing is a longitudinal cross-sectional view showing one end of an improved rotor member made in accordance with my invention.

As shown in the drawing, the rotor member comprises a spider l which supports punchings 2 held between steel end plates 3. The punchings 2 are perforated to receive standard squirrel-cage bars 4 of the usual shape common to squirrel-cage rotor meinbers. y

My invention relates/to the end rings for short-circuiting the squirrel-cage bars. I provide an outer, high-resistance end ring 5 around the outside circumference of the bars 4, taken collectively, and connected thereto, said outer end ring 5 having a high resistance suitable for low-current, high-torque starting; and an inner, low-resistance end ring 6, located within the inner circumference of said bars, taken collectively, and connected thereto, said inner end ring 6 having a low resistance suitable for low-slip operation under running conditions.

The inner end ring 6 is disposed adjacent to the iron end plate 3 of the rotor core, the end plate being of such limited outer diameter that it does not come into the vicinity of the outer end ring 5. In order to increase the reactance of the inner, low-resistance end ring 6, according to my invention, I provide an angular iron ring 7 having a channel 8 which receives the inner, low-resistance end ring 6, the channeled iron ring being permanently clamped to the iron end plate 3 by means of lag bolts 9.

In a standard squirrel-cage induction motor designed for low-slip operation but with poor starting torque, the reactance of the end ring may be live or six times the resistance of the end ring at the moment of starting. B'y enclosing, or partially enclosing, the end ring in iron, as in my invention, this reactance may be increased to three or four times its former value.

From the foregoing description and explanation, it will be seen that, by the simple expedient of providing an outer, high-resistance end ring and an inner, low-resistance end ring, and increasing the reactance of the inner, low-resistance end ring, I have provided a very effective means whereby the larger portion of the starting current may be forced through the outer, high-resistance end ring at the time of starting.

When the motor o crates from standstill to full-running speed the frequency of the current in the squirrel-cage member decreases from the full line frequency, such as cycles, to the slip frequency, such as 2 cycles, with a corresponding decrease of the reactance of the squirrel-cage circuits, so that, under running conditions, the reactance of the inner, low-resistance end ring is no longer suficient to force the major portion of the current into the outer, high-resistance end ring.

If desired, in cases where it happens, by chance or design, that the low-resistance end ring has a higher reactance than is necessary to secure a certain guaranteed minimum starting current, the outer edge of the iron channel ring 7 may be turned down, as indicated at l0, to lower the reactance of the running winding.

In general, the resistance of the high-resistance end ring should be at least several times the resistance of the low-resistance end ring, and the low-resistance ring should have a reactance which, at starting, is higher than the total impedance of the high-resistance end ring, so as to force most of the startingcurrent through the high-'resistance ring, but the reactance of the low-resistance winding should be so low, at full speed, and the resistance of the high-resistance winding should be so high, that the total impedance of the low-resistance end ring, under running conditions, is less than the resistance of the high-resistance end ring,so that mostl of the running current is forced into the low-resistance end ring.

The provision of the iron channel member -f 7 i around the inner, low-resistance end ring does not materially lower the power factor of the motor, as a whole, because the end-ring reactance in a normal motor is not more than 2 or 3 per cent of the total reactance of the l machine, so that a three-fold increaseV in the end ring vreactance will not produce more than about a 10 per cent increase in the reactance of the machine, which is as good as can be done in any special Awinding or construction which aiins to secure a low starting current with a high starting torque and without materially increasingrthe full load slip.

Whilel have shown my invention in what is believed now to be the most desirable construction, all Vthingsconsidered, it is to be understood that such showing ismerely illustrative and that any means for permanently surrounding, or 'partially surrounding, the

low-resistance' end ring with iron may -be utilizedV without departing` from the broader spirit of my invention, whether the low-resistance endring is inside or outside of the squirrel-cage-bars', and also regardless of the vpositionof the high-resistance end ring, so ilong as it is spacedsoinewhat from the iron that surrounds the low-resistance end ring.

I claim as my invention: 11. A squirrel-,cage induction-motor rotor including squirrel-cage bars, a high-resistanceend ring connected thereto, having a high .resistance suitable for low-current, hightorque starting, a low-resistance end ring convnected thereto having a low resistance suit- Y V:able forlow-slipo'peration under running conditions, and iron members iXedly disposed adjacent toV said low-resistance end ring to greatly increase the reactance thereof without materially increasing the reactance of the j high-resistance end ring, the resistancesl of the ,two end rings and the reactance of the low- 4resistance end ring being such that the highresistance'ring is effective at the moment of starting and largely ineective during run- Y ning conditions, characterized by the highresistance end ring having at least several times the resistance of the low-resistance end ring, and the low-resistance end ring having a reactance which, at start-ing, is higher than the total impedance of the high-resistance end ring, and yet which, at full-speed, is so low that the total impedance of the low-resist-- ance end ring `is less than the resistance of the high-resistance end rind.

2. A squirrel-cage induetion-nfiotor rotor including 4Vsquirrelcage bars, a high-resistance end ring around the outside circumference of said bars, taken collectively, and connected thereto, having a high resistance suitable for low-current, high-torque starting, a low-resistance end ring located within the inner circumference ofV said bars, taken collectively, and connected thereto, having a low resistance suitable for low-slip operation under Y'running conditions, and iron members dis- Yend ring, and the low-resistance end ring hav-"- ing a reactance which, at staiting,`isrhigher than the total impedance of'the high-resistance end ring, and yet which, at full-speed, isso low that the total impedance of the lowresistance. end ring is less thanthe resistance of the high-resistance end ring.

3. A squirrel-cage induction-motor rotoi including squirrel-cage bars, a high-resistance end ring connected thereto, a low-resistance end ring connected thereto at the same end j Vof the rotor, an iron end plate disposed atthat end of the' rotor core, the low-resistance end .ring being disposed adjacent to said-iron end plate, and a channeled iron ring rigidly clamped to said iron end plate, with the low-v resistance endring'rdisposed in its channel, theV high-resistance end `ring ,being spaced from said iron members.

t; A squirrel-cage induction-motor rotor including squirrel-cagebars, a high-resistance end ring around the outside circumferenceof said bars, talren collectively, and connected thereto, having a high resistance suitablefor low-current, high-torque starting, alow-*re-` sistance end ring locatedwithin the inner circumference of said bars, taken collectively, andV connected thereto,fhaving a low resistance suitable .for low-slip operation under ruiming conditions,'an iron end plate disposed adjacent tothe inner side of the low-resistance end ring but not the Vhigh-resistaiice ring, and a. channeled iron ring permanentlyV clamped 'to saidriron end plate,l with the low-resistance end ring -in its channel..

5. A squirrel-cageV inductionmotor secondwinding adapted to have ahigh effective resistance at starting, and a low effective resistance Vat full-speed, the change in resistance being all in the end connections, characterized yby having twoend connections at each end of the squirrel-cage bars, oneend connection being of the desired highv resistance for starting, and the other being lof at least severaltiines vlower resistance for'rnnning-the'high-resistance end connection 'hav-Y ing a relatively low reactance, as compared to its resistance, at starting, and the lowresistance end connection having a reactance which at starting is higher thanthe total impedance of the high-resistance end connection, and yet which, at full-speed, is so low that the total impedance of the low-resistance end connection is less than the resistance of the high-resistance end connection.

6. A squirrel-cage induction-motor rotor including squirrel-cage bars, a high-resistance end ring around the outside circumference of said bars, taken collectively, and connected thereto, having a high resistance suitable for low-current, high-torque starting, a low-resistance end ring located within the inner circumference of said bars, taken collectively, and connected thereto, having a lovv7 resistance suitable for low-slip operation under running conditions, and a thick iron channel member having an annular channel in which said low-resistance end ring is nested, the iron Walls of said channel member being of suilicient thickness to produce substantial eddy-current effects at starting, characterized by the high-resistance end ring having at least several times the resistance of the lowresistance end ring, and the low-resistance end ring having a reactance which, at starting, is higher than the total impedance of the high-resistance end ring, and yet Which, at full-speed, is so low that the total impedance of the low-resistance end ring is less than the resistance of the high-resistance end ring.

7. A squirrel-cage induction-motor rotor including squirrel-cage bars, a high-resistance end ring around the outside circumference of said bars, taken collectively, and connected thereto, havin a high resistance suitable for 10W-current, igh-torque starting, a 10W-resistance end ring located Within the inner circumference of said bars, taken collectively, and connected thereto, having a low resistance suitable for low-slip operation under running conditions, and thick iron material disposed in close proximity to at least two sides of said low-resistance end ring, the thickness of said iron member or members being sufficient to produce substantial eddy-current effects at starting, characterized by the high-resistance end ring having at least several times the resistance of the low-resistance end ring, and the lowresistance end ring having a reactance which, at starting, is higher than the total impedance of the high-resistance end ring, and yet which, at full-speed, is so low that the total impedance of the low-resistance end ring is less than the resistance of the high-resistance end ring.

In testimony whereof, I have hereunto subscribed my name this seventh day of December 1928.

ROBERT R. SHEELY. 

